Magnetostrictive linear displacement transducers are used in a wide variety of applications, primarily in industrial applications involving the provision of information as to the piston position in a hydraulic cylinder. The transducer indicates whether the cylinder is fully extended or fully retracted, and it also monitors the position throughout the entire stroke of the cylinder. Because of industrial needs in recent years for extremely precise linear positioning data in the control of manufacturing processes and in other applications, the magnetostrictive linear displacement transducer has become increasingly popular, primarily due to its accuracy and reliability and also its non-contact and wear-free method of measurement.
The magnetostrictive linear displacement transducer includes three principal components. These are a waveguide along which signals are propagated at ultrasonic velocity, a processing head at one end of the waveguide for analyzing the information, and a permanent magnet that travels with the piston or other device which is being monitored. The waveguide is typically a wire which is enclosed in a high pressure tube. The magnet normally takes the form of a ring having four magnets embedded in it.
The transducer operates on the basis of the Joule effect which is also known as magnetostriction. Electrical pulses are propagated down the waveguide, and the magnetic field they induce, combined with the field of the permanent magnet, creates a mechanical strain pulse on the waveguide. The strain pulse is propagated at ultrasonic velocity and is converted into an electrical signal which is received by the processing head. The processing head determines the position of the magnet based on the time required for the pulse to travel the distance between the magnet and processing head at a known velocity. Knowing where the magnet is at any time provides the location of the piston to which the magnet is connected.
The processing head analyzes the data based on the assumption that the velocity of the pulses is a known and constant value. However, the velocity of an ultrasonic signal varies with temperature changes, and treating the velocity as constant can lead to errors that can be unacceptably large if there are significant temperature fluctuations. Cylinders are often used in environments where the temperature is elevated considerably, and the ultrasonic pulses travel at a slower speed as the temperature is elevated. As a result, the transducer output can be erroneous, and the error can be unacceptably large in some applications. Conversely, if the unit is used in a cold environment, the ultrasound moves faster than it does at room temperature, and this also creates errors in the transducer output.